They had to verify that it could be used to visualize localization of RNA in cells. They first checked that system by targeting GAPDH with an mCherry-tagged dCas9 and indeed saw the mCherry signal exported from the nucleus. Next, they targeted the mRNA transcripts of ACTB, TFRC and CCNA2 genes. They then compared the localization patterns observed to the patterns they saw after visualizing those same genes using FISH (fluorescence in situ hybridization.) Indeed, they saw the targeted mRNAs in the same places where it was shown using FISH. They also took care to ensure that tagging the RNA does not interfere with the abundance of mRNA or the amount of translated protein.
Source: Cell